1. Field of the Invention
The invention is generally concerned with mobile radio networks.
The present invention is more particularly concerned with a terminal for a cellular mobile radio network, such as the GSM network in particular, and even more particularly with a device for authorizing the selection of a cell in a terminal of this kind in the standby state.
2. Description of the Prior Art
To be able to receive calls in the standby state, a terminal of this kind must execute a number of functions, such as listening out for "paging" messages in particular. These messages are broadcast in each cell on a broadcast control channel characteristic of the cell.
The terminal continuously selects the optimum cell, i.e. that for which transmission quality is the highest, to listen to these paging messages.
A parameter representative of the transmission quality is therefore defined, allowing in particular for the signal level as received by the terminal in a cell on the broadcast control channel of that cell, to which end the broadcast control channel is broadcast continuously in that cell.
This parameter, known as "C1" in the case of the GSM system, for example, is described in more detail on pages 453 through 455 of "The GSM System for Mobile Communications" by Michel MOULY and Marie-Bernadette PAUTET (Europe Media Duplication--1992).
Briefly, in the GSM system, to select a cell the terminal calculates the parameter C.sub.1 for each of the various cells surrounding it, eliminates those cells for which the value of C.sub.1 obtained is below a given cell selection threshold, selected to guarantee a guaranteed grade of service to the user once a call involving the terminal has been set up in the cell selected in this manner, and retains whichever of the cells obtained in this way yields the highest value C.sub.1.
If no value of C.sub.1 above the cell selection threshold can be found, because receive levels are too low, no cell can be selected and the terminal is unable to access the network to set up calls.
An essential aim of the present invention is to reduce the number of situations in which a terminal is prevented in this way from accessing the network, because no cell can be selected, or in other words, to enlarge to some degree the coverage area of the network.